Yagi (摩羯)

Yagi 是一个自定义 AVAssetExportSession 框架，可以在对导出视频压缩效果不减弱的情况下获得更小的体积。

示例代码

let exportSession = VMAssetExportSession(asset: avasset, preset: .VMAssetExportPreset1080p)
exportSession.outputUrl = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent({file_name}) exportSession.exportAsynchronously(progress: { (progress) in
    // progress handle
}, completion: { (result) in
    // completion result handle
})

如果需要自定义的 Video & Audio Settings，请使用如下方法:

func updateVideoSetting(_ videoSettings: [String: Any])
func updateAudioSetting(_ audioSettings: [String: Any])

注意: Yagi 并不能保证在自定义导出参数的情况下可以正常导出视频，同时自定义导出参数会覆盖预设参数的值，预设参数参考下面。

预设压缩参数

预设压缩参数参考了 Video Encoding Settings for H.264 Excellence 文章的内容，具体详情如下:

VMAssetExportPreset1080pHQ

video: {
    AVVideoWidthKey: {video_file}.naturalSize.width,
    AVVideoHeightKey: {video_file}.naturalSize.height,
    AVVideoScalingModeKey: AVVideoScalingModeResizeAspectFill,
    AVVideoCodecKey: AVVideoCodecH264
    AVVideoCompressionPropertiesKey: {
        AVVideoAverageBitRateKey: 7552000.0,
        AVVideoMaxKeyFrameIntervalDurationKey: 2.0,
        AVVideoAllowFrameReorderingKey: false,
        AVVideoProfileLevelKey: AVVideoProfileLevelH264High41,
    }
},

audio: {
    AVFormatIDKey: kAudioFormatMPEG4AAC,
    AVSampleRateKey: 44100.0,
    AVNumberOfChannelsKey: 2,
    AVEncoderBitRateKey: 128000.0
}

VMAssetExportPreset1080p

video: {
    AVVideoWidthKey: {video_file}.naturalSize.width,
    AVVideoHeightKey: {video_file}.naturalSize.height,
    AVVideoScalingModeKey: AVVideoScalingModeResizeAspectFill,
    AVVideoCodecKey: AVVideoCodecH264
    AVVideoCompressionPropertiesKey: {
        AVVideoAverageBitRateKey: 4992000.0,
        AVVideoMaxKeyFrameIntervalDurationKey: 2.0,
        AVVideoAllowFrameReorderingKey: false,
        AVVideoProfileLevelKey: AVVideoProfileLevelH264High41,
    }
},

audio: {
    AVFormatIDKey: kAudioFormatMPEG4AAC,
    AVSampleRateKey: 44100.0,
    AVNumberOfChannelsKey: 2,
    AVEncoderBitRateKey: 128000.0
}

VMAssetExportPreset720pHQ

video: {
    AVVideoWidthKey: {video_file}.naturalSize.width,
    AVVideoHeightKey: {video_file}.naturalSize.height,
    AVVideoScalingModeKey: AVVideoScalingModeResizeAspectFill,
    AVVideoCodecKey: AVVideoCodecH264
    AVVideoCompressionPropertiesKey: {
        AVVideoAverageBitRateKey: 3072000.0,
        AVVideoMaxKeyFrameIntervalDurationKey: 2.0,
        AVVideoAllowFrameReorderingKey: false,
        AVVideoProfileLevelKey: AVVideoProfileLevelH264High41,
    }
},

audio: {
    AVFormatIDKey: kAudioFormatMPEG4AAC,
    AVSampleRateKey: 44100.0,
    AVNumberOfChannelsKey: 2,
    AVEncoderBitRateKey: 128000.0
}

VMAssetExportPreset720p

video: {
    AVVideoWidthKey: {video_file}.naturalSize.width,
    AVVideoHeightKey: {video_file}.naturalSize.height,
    AVVideoScalingModeKey: AVVideoScalingModeResizeAspectFill,
    AVVideoCodecKey: AVVideoCodecH264
    AVVideoCompressionPropertiesKey: {
        AVVideoAverageBitRateKey: 2496000.0,
        AVVideoMaxKeyFrameIntervalDurationKey: 2.0,
        AVVideoAllowFrameReorderingKey: false,
        AVVideoProfileLevelKey: AVVideoProfileLevelH264High41,
    }
},

audio: {
    AVFormatIDKey: kAudioFormatMPEG4AAC,
    AVSampleRateKey: 44100.0,
    AVNumberOfChannelsKey: 2,
    AVEncoderBitRateKey: 128000.0
}

VMAssetExportPreset480p

video: {
    AVVideoWidthKey: {video_file}.naturalSize.width,
    AVVideoHeightKey: {video_file}.naturalSize.height,
    AVVideoScalingModeKey: AVVideoScalingModeResizeAspectFill,
    AVVideoCodecKey: AVVideoCodecH264
    AVVideoCompressionPropertiesKey: {
        AVVideoAverageBitRateKey: 1216000.0,
        AVVideoMaxKeyFrameIntervalDurationKey: 2.0,
        AVVideoAllowFrameReorderingKey: false,
        AVVideoProfileLevelKey: AVVideoProfileLevelH264High41,
    }
},

audio: {
    AVFormatIDKey: kAudioFormatMPEG4AAC,
    AVSampleRateKey: 44100.0,
    AVNumberOfChannelsKey: 2,
    AVEncoderBitRateKey: 64000.0
}

VMAssetExportPreset360p

video: {
    AVVideoWidthKey: {video_file}.naturalSize.width,
    AVVideoHeightKey: {video_file}.naturalSize.height,
    AVVideoScalingModeKey: AVVideoScalingModeResizeAspectFill,
    AVVideoCodecKey: AVVideoCodecH264
    AVVideoCompressionPropertiesKey: {
        AVVideoAverageBitRateKey: 896000.0,
        AVVideoMaxKeyFrameIntervalDurationKey: 2.0,
        AVVideoAllowFrameReorderingKey: false,
        AVVideoProfileLevelKey: AVVideoProfileLevelH264High41,
    }
},

audio: {
    AVFormatIDKey: kAudioFormatMPEG4AAC,
    AVSampleRateKey: 44100.0,
    AVNumberOfChannelsKey: 2,
    AVEncoderBitRateKey: 64000.0
}

为了防止因为视频实际尺寸小于压缩参数的目标尺寸导致压缩后出现黑边的情况，AVVideoWidthKey和 AVVideoHeightKey 参数的具体计算过程如下:

var videoNaturalSize: CGSize = videoTrack.naturalSize.applying(videoTrack.preferredTransform)
if videoNaturalSize == .zero {
    videoNaturalSize = CGSize(width: preset.width, height: preset.height)
}

var videoActualSize = CGSize(width: abs(videoNaturalSize.width), height: abs(videoNaturalSize.height))
if videoActualSize.width * videoActualSize.height > preset.width * preset.height {
    let aspectRatio = videoActualSize.width / videoActualSize.height
    let width: CGFloat = abs(aspectRatio > 1.0 ? preset.width : preset.height)
    let height: CGFloat = abs(width / aspectRatio)
    
    videoActualSize = CGSize(width: width, height: height)
}

为了防止视频实际码率小于目标码率而导致导出结果增大的情况，增加对视频实际码率的校验，如果实际码率小于目标码率时，使用实际码率作为导出参数。

var videoEstimatedBitRate = videoTrack.estimatedDataRate
if videoEstimatedBitRate == .zero {
    videoEstimatedBitRate = preset.videoBitRate
}

let videoActualBitRate = min(preset.videoBitRate, videoEstimatedBitRate)

压缩效果

被压缩视频源文件

压缩结果:

Name	Resolution	Video Bitrate (Mbps)	Audio Bitrate (kbps)	Audio Sampling Rate (kHz)	Size (MB)
源文件	3840*2160	50.2	128.0	48.0	378.7
VMAssetExportPreset1080p	1920*1080	5.0	128.0	44.1	38.6
VMAssetExportPreset720p	1280*720	2.5	64.0	44.1	19.0
VMAssetExportPreset480p	848*478	1.2	64.0	44.1	9.5
VMAssetExportPreset360p	640*360	0.9	64.0	44.1	7.2
AVAssetExportPreset1920x1080	1920*1080	14.9	128.0	48.0	114.0
AVAssetExportPreset640x480	640*360	2.6	128.0	48.0	21.8